Manufacture of anhydrous monocalcium phosphate



E. BLOCK MANUFACTURE OF ANHYDROUS MONO-CALCIUM PHOSPHATE AFiled Sept. 14, 1939 N//- LS @STM Mw Wl; JI- QQQMM mgm@ N Patented Sept. 22, 1942 MANUFACTURE F ANHYDROUS MONO- CALCIUM PHOSPHATE Edward Block, Joliet, Ill., assignor to Blockson Chemical Co., Joliet, Ill., a corporation of Il- Application September 14, 1939, Serial No. 294,826

l2 Claims.

The present invention relates to the manufacture of a mono-calcium phosphate anhydrate.

There is a demand for a mono-calcium phosphate anhydrate as a substitute for the same salt as the monohydrate. In baking powders, self-rising flour, and other prepared flour-base compositions for pancakes, waiiles, doughnuts, cakes and the like, an anhydrous form is preferred. It has a slower rate of reaction upon the bicarbonate of sodium, so that a better product is obtained. Diiiiculties have been encountered heretofore in making an anhydrous form having such qualities at a cost permitting competition with the monohydrate form. New processes for its manufacture have long been sought.

In experimenting with a new process-which has been developed and is herein described, for making anhydrous mono-calcium phosphate, I have-discovered that one of the factors of the product, namely, anhydrous di-calcium phosphate, must have special physical characteristics in order for said process to operate. 'I have found that some forms yield mono-calcium phosphate monohydrate, and that other forms yield mono-calcium phosphate anhydrate. I have also discovered that the chemical forms of the two classes of such factor are identical, as indicated byl their X-ray spectra. I have also discovered how the operable forms may be produced. Because the same procedure produces different products according to the physical type of the anhydrous di-calcium phosphate employed, I have also a new process for making anhydrous mono-calcium phosphate.

The present invention has for its objects, the production of anyhdrous mono-calcium phosphate, and the production of anhydrous dica` cium phosphate for use in the` production of tl .e said anhydrous mono-calcium phosphate.

Other objects of the invention are the provision of a simple process utilizing commercially concentrated phosphoric acid for the direct production of anhydrous mono-calcium phosphate, and the provision of a form of anhydrous dicalcium phosphate which is reactive with such phosphoric acid for such purpose.

Various other and ancillary objects' and advantages of the invention will become apparent from the following description and explanation gvenmerely to illustrate the nature of the invention, and not in limitation thereof.

I have discovered that a strong phosphoric acid 4solution in water and di-calcium phosphate anf hydrate may be reacted under controlled conditions to produce a mono-calcium phosphate which is either the anhydrate or the monohy drate according to the physical character of the di-calcium phosphate anhydrate. It is known that the monohydrate may be readily secured, but so far as I am aware it is not known that the anhydrate may be secured. Numerous factors are involved. Stated generally, there is a temperature factor, such that mere increase in temperature causes the anhydrate to be formed from materials which at lower temperature will form the monohydrate. other factor is the form of the di-calcium ph hate anhydrate. With respect to this factor, it may be said that one form (A) will give the hydrated product under the very conditions which cause another form (B) to produce the anhydrous product.

Herein the designation A refers broadly4 to inoperable forms, and the designation B refers broadly to operable forms. The present description contemplates definition of limits characterizing form B."

Therefore, in order to explain the reaction it must be shown that there is a diiference in form, and it mustv be described what form of di-calcium phosphate anhydrate is required. This is done i'lrst by teaching how a s ingle'raw materialmay be treated to produce either the` form A or B r above referred to.

In order to illustrate the nature of the invention I give first a description of a form of dicalcium phosphate dihydrate having the formula CaHPOaZI-IaO, which maybe used to produce form A or form B.

Table 1 gives the chemical analysis of the said 4 material dried at 50 C. Table 2 gives the probable composition, making useful the infomation l of Table 1. Table 3 gives the particle size distribution and apparent density as the best indication of physical size. Following this lis other information pertinent to identity.

TABLE 1..--1Chefm'cal afialyss Per cent Tum: 3.-Screen analysis Apparent density, 42 lbs. per cu. ft

Microscopic examination shows each particle t be composed of a myriad of very small crystals of di-calcium phosphate dihydrate cemented together to form an irregularly shaped particle. The X-ray pattern corresponds to that attributed to di-calcium phosphate dihydrate.

Per cent Larger than 140 mesh 7 140 to 200 mesh 19 zoo so 25o mesh ze 5 Smaller than 250 mesh. A 48 34% free water, referred to herein as the wet base.

In a vessel a 2-liter capacity, equipped for mechanical agitation, 800 grams of the said wet base is mixed to a uniform slurry with 'an equal Weight of water, and made slightly acid with 2 cc. of 85% phosphoric acid. The ilask is heated gradually and at a constant rate, the latter being done to give the following observations the explanatory significance attached to them. The course of the reaction has been timed. the pH observed, the liquid phase analyzed, and the solid phase analyzed, withl results given below in Table 4.

TABLE 4,-0baermtiom on operalion of preferred procedure Liquid phase Solid phase lginutets Temperapil ol D led A me o ure, so ur at ppal.'- heating degrees C. tion (136:35 Glls Glffs Pegnt gr? 105 d enigt percen ensl y 100 cc. 100 Ycc. 100 cc. released P10; PO an. dried Before acid.. 7. 62 42 After acid." 5. 55

24 45 60 75 6. 15 95 6. 00 044 Trace-- 102 13. 5 40. 8 44. 4 42 100. 4. 93 O90 Trace. 213 27. 7 40. 8 45. 1 42 100. 5 4. 03 113 02 309 34. 8 41. 5 '46. 3 42 g3. 5 3. 61 195 06 590 60. 0 42. 4 47. 7 42 98 97 96 95 94. 5 94. 0 94. 5 gg 3. 241 09 792 74. l 50. 5 50. 5 73 97 98 99 100. 5 100. 5 2. 91 205Y 03 703 81. 5 50. 4 50. 3 75 'I'he material above described may be made as described in Block and Metziger U. S. Patent No. 2,078,627, which yields directly a wet mass of the product.

It; is known that a water suspension of dicalciumphosphate dihydrate may be converted to anhydrous form in water by heating the sus'- pension. The change takes place at about '70 C. The form so produced is not suitable for the present invention. In accordance with the present invention, I may carry the treatment, further to produce either a form A, which is light and iluiiy, or a form B, which is heavy.

Production of form A added after reaching C., the crystals still shatter into a light iiuliy product. Production of lorm'B ini-calcium phosphate aimante o f the form described above either wet or dry may be used, As produced according to said Patent No.

2,079,627, it may have 55% dmydratesnds. aus 75 The nal product form B air-dried has a chemical analysis as in Table 5, a probable composition as in Table 6, and a particle size distribution and apparent density as in Table 7.

. TABLE 5.-Form B-Chemical analysis Per cent cao 39.4 P205 50.3 NagO 0.15 A Trace FezO: Trace S01 0.25 H2O (at 900 C.) 8.95

TAM.: 6.--Form B-Probable composition Per cent Di-calcium phosphate anhydrous CaHPO4- 96.3 Disodium phosphate NagHIPOi 0.35 Calcium'sulfate CaSO4.2H2O .55 Water, free or combined 1.85

Tnx.: 7.--Eorm`B-Screen analysis I Per cent Larger than mesh .f. 5.0 140 to 200 mesh. 16.6 200 to 250 mesh. 33.4 Smaller than 250 mesh. 45.0A

Apparent density,- 75 pounds per cu.

Microscopic examination and measurement show well formed at translucent crystals of' oblique parallelopiped form. The majority of the individual crystals fall within the measurements as follows:

Small size-40 x 65 x 15 microns Large size- 90 x 130 x 30 microns Larger particles than above appear to be the resultof several crystals sticking together. There are practical-ly no crystals smaller than the above lower limit. The X-ray pattern is that attributed Discussion of Table 4 The results of Table 4 have been chosen to explainv the invention, by specially operating the process at those conditions which experience dictates are the preferred ones. Variations are permitted, as will be explained, and of course the character of the data in Table 4 will be different.

The amount of acid present is suflicient to give the solution a pH of about 2, but this is not indicated at the start. However, as the process goes on, the pH is lowered, approaching pH of 2. There is no indication of chemical reaction utilizing the acid.

Because the wet base employed has crystals which are'agglomerates,` it is believed that these crystals absorb or adsorb the said acid. It also is shown that sodium, calcium and P205 increase in the liquid phase indicating the release of the probable di-sodium phosphate as the base material changes to more consolidated integral anhydrous form. The analyses indicate a decrease in dsodium phosphate content. Also, iron and aluminum impurities of the base pass into the liquid phase.

Following the time and temperature relation,-

and noting the regular rate of heating (and also of radiation to lose heat) the temperature is an indication of transition in the base. It is noted v that in from 65 to 'l2 minutes a maximum teminto a single unitary crystal of anhydrous dicalcium phosphate with release of impurities in the agglomerate-particle. Where the agglomerate-particle is initially large it 'appears to result in the formation of two or more integral crystals united or cemented together. Where there are very small agglomerate-particles it appears 'that vin transformation there is a tendency for them to attach' to and become integral with other crystals in their growth. Microscopic examinations made at the 'i2-minute period show particles which are part agglomerate, like 'the base, and part integral crystal, like the final product.

There is a leveling process as to particle size,.

tending to produce crystals within a quite denite range of particle size. Both the smaller and the larger particles in transition form larger and smaller particles. This is demonstrated by dividing the base into graded sizes, and repeating the process (as in Table 4) with the graded sizes. Then the products are graded. So doing this.

the following is found:

TABLE 8.--Transition size change Within the limits tested it may be said that as the particles are smaller more of them' remain the same size, more of them become coarser, and

perature is reached', that a cooling then occurs,

and a heating follows. The reversal occurs at 94 C., which is a point quite regularly observed in many experiments to be the transition temperature. If heating is very slow, a visible transition begins when the temperature of 94 C. is attained. Visible changes occur as this cooling goes on, and when at 85 minutes the temperature rises again, it is found in Table 4 that the solid phase has changed. The apparent density in particular has changed from 42 to 'i3 lbs. per cu'. ft. In other words, the base has become dehydrated and consolidated. t has not denitely 4been ascertained whether the dehydration and consolidation are simultaneous effects, in the specific illustration, but in other cases it has been demonstrated that the same nal product can result from treatment eiecting dehydration, followed by consolidation. It is concluded that the process of dehydration, even in an aqueous suspension, absorbs heat from the liquid, result-` ing in the cooling.

From the knowledge of the structure of the base and of the nal dense product of dehydration, it appears that the original particle-form agglomerated minute crystals of cli-calcium phosphate dihydrate are transformed each particle nsl ' perature range from 95 C. to 96 C.

less of them become smaller. The net effect of the above .is to indicate that the agglomerateparticles ofthe base do not shatter and form smaller crystals during the .transition-process. Rather they tend to maintain their particle integrity, and even show growth or union.

Modified procedure A water suspension containing 20% by weight of the agglomerate-particles of di-calcium phosphate dihydrate and a trace of acid, such as about 1% phosphoric acid, is heated to 85 C. The mass is heated slowly to C. over a half .hour period. Then the mass is heated much slower to 96 C. by increasing the temperature one degree per half hour. The product ltered and dried is a granular dense anhydrous dicalcium phosphate of form B, with an apparent density of 60 lbs. per cu. it., 95% to 98% passing a 200 mesh screen. In the process there is no visible change up to 94 C., but there a visible change begins, which is completed in the tem- This changeldoes not take place if the heating is too fast.

Variations The last procedure represents the manufacture of a form B having a density of 60 lbs. compared to 73 to 'l5 lbs. per cu. ft. of the preferred procedure. Other variations of the processD are permitted. These may result in inefllciency, waste and lowered yield. The percentage of the solids in'the slurry is not critical, and the amount of acid used is not critical. Increaseof acid will not stop the formation of form B, but excess of-it solubilizes calcium phosphates into the liquid phase, and increases the tendency of the crystals to attach to each other. 'I'he rate of heating is not critical, and it may be slow or rapid, but it must be suiliciently prolonged to secure the dense crystals as already shown. It is suiicient to heat only to 94 C. and to maintain that temperature. This temperature-appears to be the critical one. for the consolidation. However, it appears that `where dehydration occurs first, followed by consolidation, the apparent density is less than where these processes seem to occur together. Numerous variations of the process will produce a form 'B varying from 50 to 78 lbs. per cu. ft. However, where the acid was omitted, the di-sodium phosphate did not release from the solidphase, and the product consisted of form A, showing irregularly shaped particles, less than about lf3 microns in size, having an apparent density of 40 lbs. per cu. ft. 4The agglomerate-particles of the `base appear to have shattered before or during dehydration, and not to have consolidated.

The acid added need not be limited to phosphoric acid, and may be hydrochloric, nitric or sulphuric or other strong acid. However, these introduce impurity and eventually liberate phosphoric acid, so practically, phosphoric acid is added, as it will otherwise be formed. y

In the accompanying drawing there is shown a scale IIwith inches and mircrons marked, against which there are shown three typesr of crystals. The left hand group of crystals Il are form B crystals o f 200 to 250 mesh screen.

Those marked E show the appearance of crystals ndwise, indicating their flat shape. The facial views indicatethe parallelopiped'form, and are of size.in the micron range of 65 x 100 x 25 to 90 x 130 x 30. AThe right-hand group of crystals I2 are similar, but show those passing a 250 mesh wire, and are of size in the micron range of 40 x 65 x 15 to 65 x 100 x 25. The middle group of crystals I3 represents the form A of anhydrous di-calcium phosphate produced without acid as described above. They are generally less than I3 microns, and of apparent density of about 40 lbs. per cu. ft.

The use of form' B The process comprises generally the quick mixing o f concentrated phosphoric acid solution in water and granular form B of di-calcium phosphate anhydrate. `having an apparent density of 50 lbs. or more per cu. ft. The particle size distribution of the latter is not of critical signitlcance. The temperatures of the two reactants should be such that upon mixing, the

reaction mass will have a temperature sumciently high to produce the anhydrous product, thus avoiding those lower temperatures which will otherwise produce the monohydrate.

'I'he concentration of the .acid may vary from '15% to 90%, but preferably 80% is used. This strength is -readily obtained by open pan evaporation of water from weakersolutions, the iinal boiling point being near 140 C. 'I'his acid is commonly handled in ordinary equipment which resists corrosion. Stronger acids are more diilicult to obtain by open evaporation, because as the boiling temperature increases, 4some Vof the acid is converted to pyrophosphoric acid.` However, these may be used, forming pyrophosphate impurities in the product. Concentrations lower than 80% are more diiilcult to use, requiring begins quite sharply at '78% strength.

Determination of the exact critical temperature below which the hydrate is formed, and above which the anhydrate is formed is difficult. This is in part due to the fact that upon mixing, the temperature increases because of generated heat. There is also a cooling eifect when the steam begins to be released. The amount of water in the mass after mixing seems to play some part in shifting the critical temperature. By using a Dewar flask as a reaction vessel which conserves heat, and using the ingredients of the following Example 1, the critical temperature seems to be in the range from C. to 135 C.

In carrying out the invention practically any phosphoric acid may be used which can be heated to at least the critical temperature, as 130 C. to C. The form'B di-calcium phosphate anhydrate is then heated at least to the critical temperature, where the said acid is practically at the critical temperature, thus to assure the mixture not falling below the critical temperature.

The mechanism of the reaction itself readily indicates its progress and whether or not the temperature is high enough. 4Where the reaction succeeds there is a period of iluidity, which is very short, on the order of 30 seconds, during which the reactants maybe uniformly mixed. Then the material as quickly goes through a plastic stage and sets to a dry crumbly mass. The plasticity appears to be the result of crystal formation and is attended with the liberation of steam in visible quantity. Where the reaction to form anhydrate does not proceed, as with form A, or with too low temperatures using form B, there is no period of fluidity, no plastic stage, and no comparable liberation of steam. Rather, there is a simple wetting and quick setting to a dry crumbly hydrated product.

The temperature at which the desired reaction takesfplace is well over 100 C., and is produced by the initialheating of the ingredients and the heat of reaction. Some heat is lost in the steam. It appears as though the solid. and the liquid either mix, or react differently, in those cases where the desired product is obtained, compared to the cases where it is not obtained. In carrying out the process with a solution of 80% phosphoric i acid, the latter may be heated to its boiling point of near C., and the form B may be heated to C. The 20% water in the acid is not readily boiled away at 140 C. After quickly mix.

Excess of acid The reaction empirically is the addition 'of one 'mole of orthophosphoric acid /to one mole of dicalcium phosphate anhydrate. When the above described reaction is carried outwith equal molecular parts of the reacting ingredients, the

iluidity may be such that the mixing may not be completed within the num period. This depends upon skill or mechanical equipment. The uidity may be increased by using/an excess of acid. The

characteristic setting and liberation of steam still obtain. The temperature of the'reaction mass is such as to evaporate any residual mixture of acid and water to a suitable concentration for using the excess acid in the same reaction. The excess acid permeates the crumbly product. The excess acid may be neutralized with an amount of the form- B corresponding to it for the same reaction. This may be added before the initial product cools, or after it is cold. The materials are well mixedand heated to an elevated temperature, such as 150? C. Under these conditions the excess acid and the form B react to form more of the mono-calcium phosphate anhydrate.

' In carrying out the reaction in large quantities the reaction temperature will be raised by heat of reaction more than occurs in a small mass. The beginning of reaction may be such as to form the hydrated product, and then the heat of` Example 1 One mole of form B (di-calcium phosphate anhydrate) and 1% moles of 80% phosphoric acid are heated separately to 150 C. and 140 C. respectively. 'Ihey are quickly mixed. A period of fluidity permitting thorough mixing obtains for about 30 seconds.. Then a thickening takes place about as quickly with a liberation of steam, resulting in a dry granular set mass. The excess acid in the product may be extracted with solvents for phosphoric acid which do not aiect the mono-calcium phosphate anhydrate, such as acetone, or alcohols, ethyl acetate,l and the like. However, this procedure may be avoided by a second stage of the same reaction. The product produced as above is ground and thoroughly mixed with 1A mole of form B and the mixture heated to 150 C. which assures passing the critical temperature. Then the product is ground. It consists of 95% or more of mono-calcium phosphate anhydrate in granular, non-hydroscopic form.

Example 2 One mole of form B (di-calcium phosphate anhydrate) at 150 C. and one mole of 78% phosphoric acid heated Vto near its boiling point are quickly and well mixed. A uid stage quickly concentration is much over 80% phosphoric acid equivalent, and water (by weight) produced by atmospheric boiling down of phosphoric acid. 'lhe term phosphoric acid as used herein signiies the ortho variety of the formula H zPOi. By using a concentrated phosphoric acid near `80%, the same maybe produced economically without providing any substantial contamination by pyrophosphoric acid. It is not intended to convey the idea that all phosphoric acid solutions over 80% are contaminated with pyrophosphoric acid. That occurs only where the solutio has been obtained by use of temperatures higher than that which will produce an 80% solution by openvevaporation at normal Aatmosl pneric pressure. However. we prefer not to use y such higher strength acids, as they call for use my prior application Serial No. 266,546, filed April 7, 1939, Aand is related to my copending coiiled application Serial No. 294,825 in which is claimed the form Band the method of its production hereiny described.

I claim:

1. The method of making a monocalcium phosphate anhydrate which comprises rapidly and thoroughly mixing one mole of di-calcium phosphate anhydrate in the form of flat integral parallelopiped crystals having an apparent densityl of at least' 50 lbs. per cu`t. ft. with from 1 to 11A, moles of concentrated aqueousphos- Iphoric acid solution of strength from '75% to 90%, said materials being so heated that the temperature of the mix is above a critical tempasses into a plastic stage and then to a setting stage, with liberation of steam. 'Ihe product is merely ground and is largely mono-calcium phosphate anhydrate.

-The product The product used in self-rising sour with so? perature from C. to 135 C., whereby under said conditions a short time period of fluidity exists permitting said rapid and thorough mixing, followed quickly in succession by plasticity and setting with the liberation of steam.

2. The method of making a mono-calcium `phosphatel anhydrate which comprises rapidly and thoroughly mixing one moleof di-calcium phosphate anhydrate in the form of fiat integral parallelopiped crystals having an .apparent density of at least 50 lbs. per cu. ft. and one mole of concentrated aqueous phosphoric acid solution of strength from 75% to 90%, said materials being so heated that the temperature of the mix is above a critical temperature from 130 C. to

C., whereby under said conditions a short time period of fluidity exists permitting said rapid and thorough mixing, followed quickly in succession by plasticity and setting with the' liberation of steam. y

3. The method of making mono-calcium phosphate anhydrate which comprises rapidly and thoroughly mixing one mole of di-calcium phosphate anhydrate in the formof flat integral parallelopiped crystals having an apparent densityl of at least 50 lbs. perou. ft. and in excess up to 25% of one mole of concentrated aqueousv phosphoric acid o! strength from 75% to 90%, said materials with the resulting product a quantity of said di-V calcium phosphate anhydrate molecularly equivaient to the excess acid in said product, and

' heating to a temperature above said critical temperature.

4. The method of making mono-calcium phosphate anhydrate which comprises rapidly and thoroughly mixing one mole of` di-calcium phosphate anhydrate in the form of :dat integral parallelopiped crystals having an apparent density of at least 50 lbs. per cu. ft. which is heated to at least about 130 C. tov 135 C, and from 1 to 1% moles of concentrated aqueous phosphoric acid solution which is similarly heated at atmospheric pressure, whereby the mixture has a temperature above the critical temperature for forming the fanhydrate in preference to the monohydrate, the di-calcium phosphate anhydrate being that form which is capable under said conditions of permitting upon mixing a short time period of 'iluidity permitting said rapid and thorough mixing, followed quickly in succession by plasticity and settingwith liberation of steam.

5. The process which comprises reacting at least one mole of concentrated aqueous phosphoric acid solution boiling at atmospheric pressure above a critical reaction temperature around 130 C. Ato 135 C. with one mole of a form of di-calcium phosphate anhydrate having an apparent density of at least 50 lbs. per cu. ft. which is reactive with said acid above said critical temperature to form solid mono-calcium phosphate anhydrate, and reactive with said acid below said critical temperature to form mono-I calcium phosphate mono-hydrate, the quantity of acid being insuilicient to form a homogeneous solution during the reaction.

6. The process which comprises reacting conphate anhydrate in the form of flat integral parallelopiped crystals having an apparent density of at least 50 lbs. per cu. ft. and in excess up to 25% of one mole of concentrated aqueous phosphoric acid of strengtlr from 75% to 90%,

said materials being so heated that the temperature of the mix is above a critical temperature from 130 C. to 135 C., whereby under said conditions a short time period of iluidity exists permitting said rapid and thorough mixing, followed quickly in succession by plasticity and setting with the liberation of steam, and'thereafter removing excess phosphoric acid as such from the presence of said mono-calcium phosphate.

9. The method of making a mono-calcium phosphate anhydrate which comprises rapidly and thoroughly mixing one mole of di-calcium phosphate anhydrate in the form of fiat integral parallelopiped crystals having an apparent density of at least 50 lbs. per cu. ft. with at least one mole of concentrated aqueous phosphoric acid solution of strength from '75% to 90%, said materials being so heated that the temperature of the mix is above a critical temperature from 130 C. to 135 C., and the quantity of acid being insuilicient to form a homogeneous solution during the reaction, whereby the materials react to form solid anhydrous mono-calcium phosphate.

l0. The method of making mono-calcium phosphate anhydrate which comprises rapidly and thoroughly mixing one mole of iii-calcium phosphate anhydrate in the form of flat integral parallelopiped crystals having an apparent density of at least lbs. per cu. it. and in excess of one mole of concentrated aqueous phosphoric acid of strength from 75% to 90%, said materials being so heated that the temperature of the mix is above a critical temperature from 130 C. to 135 C., and the quantity of acid being insuiiicient to form a homogeneous solution during the reaction, whereby the materials react to form solid anhydrous mono-calcium phosphate, and removing excess of phosphoric acid as such from the presence of said anhydrous mono-calcium phosy phate.

11.A The method of making mono-calcium phosphate anhydrate which comprises rapidly and thoroughly mixing one mole of di-calcium phosphate anhydrate in the form of ilat integral a crystal of 40 x 65x 15 microns and a crystal of 90 x 130 x 30 microns.

7. The method of making mono-calcium phosphate anhydrate which comprises rapidly and thoroughly mixing one mole of di-calcium phosphate anhydrate in the form of iiat integral parallelopiped crystal having an apparent density of at least 50 lbs. per cu. ft. and in excess up to 25% of one mole of concentrated aqueous phosphoric acid of strength from "15% to 90%, said materials being so heated that the temperature of the mix is above a critical temperature from 130 C. to 135 C., whereby under said conditions a short time vperiod of iiuidity exists permitting said rapid and thorough mixing, followed quickly in succession by plasticityv and setting with the liberation of steam, thereafter thoroughly mixing with the resulting product a quantity ot a di-calcium phosphate molecular-ly equivalent to the excess acid in said product whereby to neu-'- tralizc the excess of acidity forming mono-calcium phosphate. A

8. The method of making mono-calcium phosphate anhydrate which comprises rapidly andA parallelopped crystals having an apparent density of at least 50 lbs. per cu. ft. and in excess of one mole of concentrated aqueous phosphoric acid of strength from 75% to 90%, said materials being so heated that .the temperature of the -mix is above a critical temperature from C. to 135 C., and the quantity of acid being insuiiicient to form a homogeneous solution during the reaction, whereby the materials react to frm solid anhydrous mono-calcium phosphate,

and extracting'free phosphoric acid from association with said anhydrous mono-calciumphosphate by the extracting action of a volatile solvent Which is selective for the remaining phosphoric acid solution and inert to mono-calcium phosphate anhydrate.

12. The method of making mono-calcium phosphate anhydrate which comprises reacting at least one mole of a phosphoric acid HsPO4 in the form vof an aqueous solution -having from 75% to 90% by weight of said acid, with one mole of di-calcium phosphate anhydrate in a physical form having an apparent density of at least 50'1bs. per cu. It. and said reaction being conducted above a critical temperature, the

quantity of acid being insuiicient to form a. homogeneous solution during the reaction. whereby to form Solid mono-calcium phosphate anhydrate, said critical temperature being that below which the same reacting materials produce'mono-calcium phosphate monohydrate, said critical temperature being also thatabove and 5 phosphate monohydrate.

below which an identical acid solution reacted with an identical quantity of di-,calcium phosphate anhydrate of apparent density of less than 50 1bs.per cu. ft. also gives mono-calcium EDWARD BICK.

. CERTIFICATE CF CORRECTION. Patt No. 2,296,195. september 22, 1ste.'

' EDWARD BLOCK.

It is h'erlebyertified that error appers in the printed speifiation ofthe aboveVnumbexfed patent requiring Correction a's follows: Page l, first vccluxnn', line 56, for"'anyhdrous" read --aphydrous-Tg-page. 5, sec'md co1- 1;;9, claim after *integral* -inset paraile1piped; and that the-said Lette-rs Patent should be read with this correction therein Athat the samel maygigi-@rml to the record of lthe case in the Patent Office.

signdnd sealed this 10th day of`Noember, A, n. 19kg;

-Helm'y Van Arsdale, (seal) l Acting commissioner' of Patents. 

